JP2010021719A - Doherty amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To actualize a doherty amplifier suitable for various radio specifications. <P>SOLUTION: A doherty amplifier has a carrier amplifier 14 of an AB class performance and a peak amplifier 15 of a C class performance, wherein input high frequency signals are divided and supplied to the carrier amplifier 14 and the peak amplifier 15, and a combined output of the outputs from the carrier amplifier 14 and the peak amplifier 15 is impedance-converted to be outputted. A drain voltage of an FET which is an amplifying element of the carrier amplifier 14 and the peak amplifier 15 is optimized according to a center frequency of the input high frequency signal. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a Doherty amplifier that is used as a power amplifier of a transmission unit in communication apparatuses according to various wireless standards.

  In recent years, the progress of mobile communication and wireless communication has been remarkable, and for example, cellular phones, wireless LANs, Bluetooth (registered trademark) and the like have been standardized. Since these radio standards have different frequency bands and output powers to be used, there is a demand for a high-efficiency amplifier that can be shared by these radio standards. In mobile communication and wireless communication, both terminals and base stations are highly demanded to improve amplifier efficiency and linearity. Efficiency is defined by the ratio of DC power consumption and high-frequency output power. The larger this value, the lower the power consumption can be achieved. Linearity is a performance in which an increase in output power with respect to an increase in input power changes linearly. If this linearity is good, distortion of the output signal is reduced and adjacent channel leakage power is reduced.

  However, in a typical amplifier, there is a trade-off between efficiency and linearity. In particular, the efficiency increases as the input level to the amplifier increases, so that the efficiency is extremely poor when the input level is low, and the maximum efficiency cannot be obtained until the output of the amplifier approaches the saturated output power. For this reason, it is extremely difficult to realize the linearity of the amplifier while maintaining the efficiency. Therefore, development of Doherty amplifiers having higher efficiency and lower distortion characteristics by combining a technique for amplifying a signal with high efficiency and distortion compensation techniques such as low distortion and feedforward has been underway.

  Here, the Doherty amplifier has a carrier amplifier and a peak amplifier, and operates the carrier amplifier in class AB and the peak amplifier in class C by adjusting the respective gate voltages. According to this configuration, when the small signal is operated, the peak amplifier does not operate because it is a class C, and only the class AB carrier amplifier operates, so that the efficiency is improved, and when the large signal is operated, the peak amplifier also operates. Saturated output power increases while maintaining high efficiency. On the other hand, an ordinary amplifier that simultaneously outputs two amplifiers by class AB operation can obtain a saturation output power equivalent to that of a Doherty amplifier, but in a practical area where backoff is taken from the saturation output power. Low efficiency.

  However, in the conventional Doherty amplifier as described above, it is necessary to optimize the impedance so that the load impedance of the peak amplifier viewed from the output combining point of the carrier amplifier and the peak amplifier appears to be open. If not open, the power leaks to the peak amplifier side when a small signal is operated, that is, when only the carrier amplifier is operating. However, in general, the load impedance often changes greatly with respect to the frequency. For this reason, the load impedance of the peak amplifier viewed from the synthesis point may not be open depending on the frequency. Therefore, the conventional Doherty amplifier has a saturation transition point only at a certain frequency, and cannot be used in common for communication apparatuses having different frequencies.

As a conventional technique for obtaining high efficiency in a wide frequency bandwidth in a Doherty amplifier, Patent Document 1 discloses that the carrier wave of the input signal is present on the output side of the carrier amplifier, the input side of the peak amplifier, and the output side of the peak amplifier. A structure has been proposed in which an element having an optimum line length according to the frequency is used and an optimum line length is given by switching according to the frequency of the carrier wave. Patent Document 2 proposes a circuit that includes a transmission line, a capacitor, and a switch on the output side of the carrier amplifier and switches the electrical length to 90 degrees according to the frequency of the carrier wave.
JP 2006-345341 A JP 2007-019578 A

  As described above, the Doherty amplifier has conventionally proposed technologies for realizing a wide band of efficiency, but all of them require lines or circuit elements corresponding to the operating frequency, and are structurally complicated. End up.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Doherty amplifier capable of realizing high efficiency with a simple structure even when shared by wireless communication apparatuses having different operating frequencies.

  In order to achieve the above object, a Doherty amplifier according to the present invention includes a class AB operation carrier amplifier that always amplifies an input signal, a class C operation peak amplifier that amplifies the input signal when the input signal exceeds a predetermined level, and A distribution circuit that distributes and supplies an input high-frequency signal to the input terminals of the carrier amplifier and the peak amplifier, an impedance conversion circuit that performs impedance conversion on a combined output of the output of the carrier amplifier and the output of the peak amplifier, and And a control circuit that variably controls drive voltages of the carrier amplifier and the peak amplifier according to a center frequency of the input high-frequency signal.

  As described above, by optimizing the drive voltage in accordance with the center frequency of the input high-frequency signal, higher efficiency than that of a normal class AB power amplifier can be obtained at different frequencies and different output powers.

  According to the present invention, it is possible to provide a Doherty amplifier that achieves high efficiency with a simple structure even when shared by wireless communication apparatuses having different frequencies of use.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block circuit diagram showing a configuration of a Doherty amplifier that amplifies a 50Ω microwave signal as an embodiment according to the present invention. In FIG. 1, the 50Ω microwave signal supplied to the input terminal 11 is supplied to one input terminal of the 90 ° hybrid coupler 12. A 50Ω terminator 13 is connected to the other input terminal of the 90 ° hybrid coupler 12, and the coupler 12 distributes the input signal to two systems with substantially equal power. The distributed signals are sent to the carrier amplifier 14 and the peak amplifier 15, respectively. For signal distribution, a general distribution circuit such as a Wilkinson distributor can be used in addition to the 90 ° hybrid coupler.

  A bias condition is given so that the carrier amplifier 14 operates in class AB and the peak amplifier 15 operates in class C. As a result, the carrier amplifier 14 is always in an operating state, and the output power of the carrier amplifier 13 is saturated in the peak amplifier 15. It starts to work when approaching. Specifically, each of the carrier amplifier 14 and the peak amplifier 15 employs an FET (field effect transistor) as an amplifying element, and the sources of both are commonly connected and grounded. The microwave signals equally distributed by the 90 ° hybrid coupler 12 are supplied to the gates of the amplifiers 14 and 15, both receive the drain voltage from the control circuit 16, and output the drain output according to the change in the gate voltage. Amplify power.

  The output of the carrier amplifier 14 is sent to a quarter wavelength impedance converter 20 via a phase compensation line 17 and a quarter wavelength line (50Ω) 18. The output of the peak amplifier 15 is combined with the output of the ¼ wavelength line 18 on the carrier amplifier side via the phase compensation circuit 19 and sent to the ¼ wavelength impedance converter 20. Since the combined output of the carrier amplifier 14 and the peak amplifier 15 is 25Ω, the quarter-wavelength impedance converter 20 impedance-converts the 25Ω signal to a 50Ω system to match the outside. The microwave signal amplified in this way is led out from the output terminal 21 to the outside.

  In the control circuit 16, an optimum drain voltage value obtained in advance for each center frequency of the input signal is registered, and upon receiving an input mode designation (for example, a user designation such as a mobile phone mode or a wireless LAN mode) of the mounted device. The corresponding drain voltage is sent to the carrier amplifier 14 and the peak amplifier 15.

  The features of the Doherty amplifier according to the present invention in the above configuration will be described below.

  First, the operation principle of the Doherty amplifier will be described. That is, in the Doherty amplifier, the carrier amplifier 14 is operated in class AB and the peak amplifier 15 is operated in class C by adjusting the gate voltages of the carrier amplifier 14 and the peak amplifier 15 respectively. The output line of the peak amplifier 15 is set to have a phase line length so that the load impedance viewed from the combined point of the output ends of the carrier amplifier 14 and the peak amplifier 15 when viewed from the peak amplifier 15 is almost open. As a result, during the small signal operation, the peak amplifier 15 does not operate because it is class C, and only the class AB carrier amplifier 14 operates.

  Here, a normal amplifier that operates two amplifiers in class AB can obtain a saturated output power characteristic as shown by a curve C1 in FIG. The linearity in the area where the back-off is taken is poor and the efficiency is low. On the other hand, in the Doherty amplifier, the class C peak amplifier 15 does not operate and only the class AB carrier amplifier 14 operates in the small signal operation, so that the linearity is good as shown by the curve C2 in FIG. , Improve efficiency. Then, when the output power increases to a saturation transition point Pa or higher at which the carrier amplifier 14 saturates and a large signal is operated, the peak amplifier 15 is also operated. As a result, as shown by a curve C3 in FIG. The power Pb increases. Further, in the Doherty amplifier composed of the carrier amplifier 14 and the peak amplifier 15 having the same saturation output power, as shown by the curves C2 and C3 in FIG. 2, the efficiency has a discontinuity point at the saturation transition point Pa. The saturation output power Pb is reached while maintaining its efficiency substantially.

  Further, in the Doherty amplifier according to the present invention, by optimizing the drain voltages of the carrier amplifier 14 and the peak amplifier 15, the saturation output power characteristic changes in the relationship between the output voltage and the efficiency as shown in FIG. That is, when the drain voltage is adjusted to V0, V1, and V2 (V0> V1> V2), the curve C4 when V0, the curve C5 when V1, and the curve C6 when V2 is set. Thus, the saturation output power characteristic changes. Further, as apparent from FIG. 3, the relationship between the output powers P0, P1, and P2 at the saturation transition points at the time of adjusting the drain voltages V0, V1, and V2 is P0> P1> P2, respectively. The output power at is different.

  The Doherty amplifier having the above characteristics needs to optimize the impedance so that the load impedance of the peak amplifier 15 seen from the output combination point of the carrier amplifier 14 and the peak amplifier 15 appears to be open. However, in general, the impedance often changes greatly when the frequency of the transmission signal changes. For this reason, the load impedance of the peak amplifier 15 viewed from the synthesis point may not be open depending on the frequency of the transmission signal. If not open, power leaks to the peak amplifier 15 side when a small signal is operated, that is, when only the carrier amplifier 14 is operating.

  Therefore, in the present invention, the control circuit 16 optimizes the drain voltages of the carrier amplifier 14 and the peak amplifier 15 according to the frequency of the radio signal. The following describes what effect can be obtained by optimizing the drain voltage in this way.

FIG. 4 shows the load impedance characteristic of the peak amplifier 15 when the frequency of the transmission signal is changed with the drain voltage kept constant. As can be seen from FIG. 4, the load impedance at the frequency f ₀ appears almost open, but the load impedance at the frequencies f _L and f _H deviates from the open state. In this case, it operates as an ideal Doherty amplifier at the frequency f ₀ . However, since the load impedance of the peak amplifier 15 does not appear to be open at the frequencies f _L and f _H , the power amplified by the carrier amplifier 14 leaks to the peak amplifier at the synthesis point during small signal operation. As a result, the efficiency decreases.

5A, 5B, and 5C show load impedance simulation characteristics when the drain voltage V _ds of the Doherty amplifier is changed to 32V, 26V, and 20V, respectively. The simulation results of FIG. 5 are summarized in Table 1.

From Table 1, it can be seen that the frequency at which the load impedance appears almost open changes from 2.20 GHz to 2.10 GHz by changing the drain voltage V _ds from 32 V to 20 V.

32V drain voltage V _ds Figure 6 shows the simulation results of the drain efficiency / frequency characteristics when changing the 20V. In FIG. 6, A, B, C shows the drain efficiency / frequency characteristics of the respective by AB class operation drain voltage V _ds = 32V, as 20V, D, E, F, respectively drain voltage V _ds = 32V, 20V The drain efficiency / frequency characteristics when the Doherty operation is performed are shown. In FIG. 6, the efficiency is summarized by the output power that is 7 dB back-off from the saturated output power (Psat). FIG. 6 shows that when the frequency is 1.90 GHz to 2.07 GHz, a higher value than the efficiency obtained by the class AB operation can be obtained by setting the drain voltage to 20V.

On the other hand, the drain voltage V _ds 32V, 26V, simulation of the frequency characteristics of the saturated output power when changing the 20V results are as shown in FIG. As can be seen from FIG. 7, the saturation output power tends to decrease when the drain voltage is set to a low value. Therefore, it can be seen that high efficiency is obtained with different output power in each frequency band.

  As described above, by optimizing the drain voltage of the Doherty amplifier in accordance with the center frequency of the radio signal, it is possible to obtain higher efficiency than the power amplifier in normal class AB operation at different frequencies and different output power.

  FIG. 8 is an actual circuit diagram in which the Doherty amplifier shown in FIG. 1 is patterned on a substrate. The microwave signal input from the input end 11 is supplied to one input end of the 90 ° hybrid coupler 12 via the pattern line P1. The other input end of the 90 ° hybrid coupler 12 is connected to the 50Ω terminator 13 via the pattern line P2, and one output end is connected to the gate electrode G1 of the carrier amplifier 14 via the pattern line P3. The end is connected to the gate electrode G2 of the peak amplifier 15 through the pattern line P4. That is, the microwave signal supplied to the 90 ° hybrid coupler 12 is sent to one output end and the other output end with the same amplitude. The phase of the signal output from the other output end is delayed by 90 ° from the phase of the signal output from the one output end.

  The output of the carrier amplifier 14 is supplied to the synthesis point via the drain electrode D1 and the pattern line P5. The pattern line P5 functions as the phase compensation line 17 and the quarter wavelength line 18 shown in FIG. On the other hand, the output of the peak amplifier 15 is supplied to the synthesis point via the drain electrode D2 and the pattern line P6. The pattern line P6 functions as the phase compensation line 19 shown in FIG. The signal synthesized at the synthesis point is output from the output terminal 21 via the impedance converter 20 by the pattern lines P7 and P8 having different line widths.

  For the Doherty amplifier configured as described above, when the frequency of the radio signal is designated, the control circuit 16 generates a drain voltage corresponding to the designated frequency and outputs the drain voltage to the carrier amplifier 14 and the peak amplifier 15. As described above, according to the present invention, drain control can be easily realized even in the case of an actual circuit configuration, and the optimum drain voltage can be switched in accordance with the operating frequency.

  The Doherty amplifier of the present invention can be applied to a power amplifier used in a radio base station and a terminal such as a mobile phone and WiMAX.

  The present invention is not limited to the above embodiment. For example, in the above-described embodiment, the drain voltage is variably controlled in the control circuit 16 based on the frequency designation of the radio signal, but the center frequency of the input signal is automatically set using a frequency detector (not shown). If the drain voltage is variably controlled according to the detection result, an input designation operation becomes unnecessary, which is more effective.

  Further, the present invention can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Furthermore, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment.

1 is a circuit diagram showing a configuration of a Doherty amplifier according to an embodiment of the present invention. The figure which shows the load impedance of the Doherty amplifier of FIG. The figure which shows the load impedance at the time of changing the drain voltage of the Doherty amplifier of FIG. The characteristic view which shows the frequency characteristic of the efficiency in the Doherty amplifier of FIG. The characteristic view which shows the frequency characteristic of the saturation output electric power in the Doherty amplifier of FIG. FIG. 2 is a pattern diagram showing a pattern formation example of the Doherty amplifier shown in FIG. 1. The figure regarding the efficiency characteristic of Doherty amplifier. The figure which shows the efficiency characteristic of a Doherty amplifier at the time of making drain voltage variable.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Input end, 12 ... 90 degree hybrid coupler, 13 ... 50ohm termination | terminus, 14 ... Carrier amplifier, 15 ... Peak amplifier, 16 ... Control circuit, 17 ... Phase compensation line, 18 ... 1/4 wavelength line (20ohm), DESCRIPTION OF SYMBOLS 19 ... Phase compensation circuit, 20 ... 1/4 wavelength impedance converter, 20 ... 1/4 wavelength impedance converter, 21 ... Output end.

Claims (4)

A carrier amplifier that constantly amplifies the input signal;
A peak amplifier that amplifies the input signal when it exceeds a predetermined level;
A distribution circuit that distributes an input high-frequency signal to the input terminals of the carrier amplifier and the peak amplifier;
An impedance conversion circuit that outputs a combined output of the output of the carrier amplifier and the output of the peak amplifier by impedance conversion;
A Doherty amplifier comprising: a control circuit that variably controls drive voltages of the carrier amplifier and the peak amplifier according to a center frequency of the input high-frequency signal. The amplification elements of the carrier amplifier and the peak amplifier are FETs (field effect transistors),
The source of each of the carrier amplifier and the peak amplifier is connected to a reference potential line,
The high frequency signal distributed by the distribution circuit is supplied to the FET gates of the carrier amplifier and the peak amplifier, respectively.
2. The Doherty amplifier according to claim 1, wherein a drain voltage variably controlled by the control circuit is supplied to a drain of each of the carrier amplifier and the peak amplifier.   The control circuit is provided with a table indicating in advance an optimum drive voltage value for each center frequency of the input high-frequency signal, and is supplied to the carrier amplifier and the peak amplifier with reference to the table when the center frequency of the input signal is designated 2. The Doherty amplifier according to claim 1, wherein the voltage value is determined.   The control circuit includes a table that shows in advance an optimum drive voltage value for each center frequency of the input high-frequency signal, and frequency detection means for detecting the center frequency of the input high-frequency signal, and the frequency detection is performed with reference to the table. 2. The Doherty amplifier according to claim 1, wherein an optimum driving voltage value corresponding to the center frequency detected by the means is determined.

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